An IPQDF Technical Resource

Introduction

In rural and agricultural settings, three-phase power is often unavailable. Yet many applications—irrigation pumps, grain dryers, livestock operations—require high horsepower (10-100+ HP). This creates a unique engineering challenge: how to deliver substantial mechanical power from a single-phase electrical supply.

Three distinct technologies have addressed this challenge over the past century:

Each has its place in history and modern practice. This guide explores all three.

flowchart TD
    subgraph Challenge["THE CHALLENGE: Rural Single-Phase Power"]
        C1[No Three-Phase Available<br>Farm, Remote Location]
        C2[High Power Required<br>10-100+ HP for Pumps, Grain, Irrigation]
    end

    subgraph Solutions["TECHNOLOGY SOLUTIONS"]
        S1[ROSENBERG MOTOR<br>1910s-1950s<br>Historical - Obsolete]
        S2[WRITTEN-POLE MOTOR<br>1990s-Present<br>Modern - Low Starting Current]
        S3[VFD + PHASE CONVERTER<br>1980s-Present<br>Variable Speed - Needs Harmonics Mitigation]
    end

    subgraph Selection["SELECTION GUIDE"]
        D1[New Installation? → Use Written-Pole or VFD]
        D2[Existing Rosenberg? → Maintain or Retrofit]
        D3[Variable Speed Needed? → VFD + Converter]
        D4[Weak Grid? → Written-Pole Preferred]
    end

    Challenge --> Solutions
    Solutions --> Selection

    style Challenge fill:#e1f5fe,stroke:#01579b,stroke-width:2px
    style Solutions fill:#fff3e0,stroke:#e65100,stroke-width:2px
    style Selection fill:#e8f5e8,stroke:#1b5e20,stroke-width:2px
    
    style S1 fill:#ffebee,stroke:#b71c1c
    style S2 fill:#e8f5e8,stroke:#1b5e20
    style S3 fill:#f3e5f5,stroke:#4a148c
    
    style D1 fill:#f3e5f5
    style D2 fill:#ffebee
    style D3 fill:#e1f5fe
    style D4 fill:#e8f5e8

Diagram created by IPQDF.com – Original work

Part 1: The Rosenberg Motor (Historical Context)

1.1 Overview

The Rosenberg Motor (also known as the Steinmetz-Rosenberg Motor) is a historic single-phase AC motor design developed by Charles Proteus Steinmetz and E.J. Rosenberg at General Electric in the early 1900s. It was engineered to solve a specific problem: delivering high horsepower (up to 100 HP) from single-phase power supplies in rural areas without three-phase infrastructure.

While obsolete and no longer manufactured, these motors may still be encountered in vintage installations. Understanding them is useful for:

• Maintaining legacy equipment
• Historical perspective on motor design
• Appreciating modern solutions like Written-Pole and VFD technology

1.2 Key Innovation: Inductor Winding

The Rosenberg motor’s main contribution was a stationary inductor winding that improved power factor and reduced brush sparking compared to earlier repulsion motors.

1.3 Operating Principle Summary

The motor operated in two modes:

1. Starting (Repulsion Mode): High starting torque (300-400%) with moderate starting current (3-5x FLC)
2. Running (Induction Mode): After centrifugal switch activated at ~75% speed, ran as induction motor

1.4 Why It’s Obsolete

1.5 If You Encounter One Today

Do not install a Rosenberg motor in a new application. If maintaining an existing installation:

• Inspect brushes and commutator regularly
• Keep spare brushes if available
• Plan for replacement with Written-Pole or VFD system
• Document for historical interest

1.6 Quick Facts

Part 2: The Written-Pole Motor (Modern)

2.1 Overview

The Written-Pole Motor is a modern single-phase, constant-speed synchronous motor designed specifically for high-inertia loads on weak rural grids. Developed by Precise Power Corporation in the 1990s, it represents a fundamental rethinking of how to start heavy loads without disturbing the power system .

The name comes from its unique operating principle: magnetic poles are “written” onto the rotor surface while it rotates, allowing extremely gentle starting and excellent voltage dip ride-through .

flowchart TD
    subgraph Stator["STATOR ASSEMBLY"]
        Main["Main Winding<br>Single-Phase AC"]
        Exciter["Exciter Winding<br>Magnetic Writing Coil"]
    end
    
    subgraph Rotor["ROTOR ASSEMBLY"]
        Ferro["Ferromagnetic Layer<br>'Writeable' Magnetic Material"]
        Poles["Written Magnetic Poles<br>Created While Rotating"]
    end
    
    subgraph Operation["OPERATING SEQUENCE"]
        Step1["1. START: Induction Mode<br>Low Current: 2-3x FLC"]
        Step2["2. WRITE: Exciter Writes Poles<br>Onto Rotor Surface"]
        Step3["3. RUN: Synchronous Mode<br>Constant Speed, No Slip"]
        Step4["4. REWRITE: Continuous Process<br>Auto-Resynchronization"]
    end
    
    subgraph Advantage["KEY ADVANTAGES"]
        A1["✓ Ultra-Low Starting Current"]
        A2["✓ Voltage Dip Ride-Through"]
        A3["✓ No Brushes - Low Maintenance"]
        A4["✓ Absorbs Grid Harmonics"]
    end
    
    Main --> Ferro
    Exciter --> Poles
    Poles --> Step3
    Step1 --> Step2 --> Step3 --> Step4
    Operation --> Advantage
    
    style Stator fill:#e1f5fe,stroke:#01579b
    style Rotor fill:#f3e5f5,stroke:#4a148c
    style Operation fill:#e8f5e8,stroke:#1b5e20
    style Advantage fill:#fff9c4,stroke:#f57f17

2.2 Why It Was Revolutionary

2.3 Construction & Operating Principle

How It Works:

1. Start as Induction Motor: The motor starts as a low-current induction motor, drawing only 2-3x full load current—dramatically less than the 6-10x of standard motors.
2. Magnetic Writing: While rotating, the exciter winding creates a magnetic field that “writes” poles onto a special ferromagnetic layer on the rotor surface. This is a continuous process—poles are written and rewritten as the rotor turns.
3. Synchronous Operation: Once poles are written, the rotor locks to synchronous speed (no slip) and operates as a true synchronous motor with constant speed regardless of load (within its rating).
4. Continuous Rewriting: The poles are continuously rewritten, meaning the motor automatically resynchronizes after disturbances—a key advantage over permanent magnet synchronous motors .

2.4 Key Performance Characteristics

2.5 The Power Quality Advantage

The Written-Pole motor’s most significant contribution to power quality is its extremely low starting current and voltage dip ride-through capability.

Starting Current Comparison

flowchart TD
    subgraph Stator["STATOR ASSEMBLY"]
        Main["Main Winding<br>Single-Phase AC"]
        Exciter["Exciter Winding<br>Magnetic Writing Coil"]
    end
    
    subgraph Rotor["ROTOR ASSEMBLY"]
        Ferro["Ferromagnetic Layer<br>'Writeable' Magnetic Material"]
        Poles["Written Magnetic Poles<br>Created While Rotating"]
    end
    
    subgraph Operation["OPERATING SEQUENCE"]
        Step1["1. START: Induction Mode<br>Low Current: 2-3x FLC"]
        Step2["2. WRITE: Exciter Writes Poles<br>Onto Rotor Surface"]
        Step3["3. RUN: Synchronous Mode<br>Constant Speed, No Slip"]
        Step4["4. REWRITE: Continuous Process<br>Auto-Resynchronization"]
    end
    
    subgraph Advantage["KEY ADVANTAGES"]
        A1["✓ Ultra-Low Starting Current"]
        A2["✓ Voltage Dip Ride-Through"]
        A3["✓ No Brushes - Low Maintenance"]
        A4["✓ Absorbs Grid Harmonics"]
    end
    
    Main --> Ferro
    Exciter --> Poles
    Poles --> Step3
    Step1 --> Step2 --> Step3 --> Step4
    Operation --> Advantage
    
    style Stator fill:#e1f5fe,stroke:#01579b
    style Rotor fill:#f3e5f5,stroke:#4a148c
    style Operation fill:#e8f5e8,stroke:#1b5e20
    style Advantage fill:#fff9c4,stroke:#f57f17

Voltage Dip Ride-Through

While standard induction motors stall when voltage drops below 80-85%, Written-Pole motors can:

• Ride through voltage sags down to 50% for 5-10 seconds
• Continue operating during dips that would trip other motors
• Automatically resynchronize after disturbances
• Reduce nuisance tripping in weak grid areas

2.6 Applications

Primary: Rural & Agricultural

• Irrigation pumps (deep-well, center pivot)
• Oil well pumps (pumpjacks)
• Grain handling (elevators, dryers)
• Dairy operations (vacuum pumps, milkers)

Secondary: Critical Infrastructure

• Standby generator sets (motor starting)
• Water/wastewater treatment (lift stations)
• Mining ventilation (remote sites)
• Telecommunications (backup power)

Tertiary: Industrial

• Large fans and blowers
• Compressors (where variable speed not needed)
• Conveyors (constant speed applications)

2.7 Advantages & Disadvantages

✅ Advantages

❌ Disadvantages

2.8 Written-Pole vs. Other Technologies

2.9 Installation Considerations

Electrical Requirements

• Dedicated single-phase supply at motor voltage
• Disconnect switch and overload protection per NEC/CEC
• Proper grounding for sensitive electronics
• Surge protection recommended for rural areas

Mechanical Considerations

• Concrete pad or sturdy base (motors are heavy)
• Proper alignment with driven equipment
• Vibration isolation if needed
• Weather protection for outdoor installations

Utility Coordination

• Notify utility before installation (especially >25 HP)
• Verify voltage regulation at site
• Consider power factor if on demand metering
• Document starting current for future reference

Part 3: VFD + Phase Converter Systems

3.1 Overview

When three-phase power is unavailable but high horsepower is needed for rural applications, a Variable Frequency Drive (VFD) combined with a phase converter (or a VFD specifically designed for single-phase input) offers a modern, flexible solution. This approach allows standard three-phase motors—which are cheaper, more efficient, and more readily available than large special-purpose single-phase motors—to operate from a single-phase supply .

Unlike dedicated single-phase motors like the Rosenberg or Written-Pole designs, VFD-based systems provide variable speed control, soft-start capability, and programmable operation—features increasingly valuable for modern agricultural and industrial applications .

3.2 How It Works: Two Approaches

Approach A: Single-Phase Input VFD + Three-Phase Motor

Some VFDs are specifically designed to accept single-phase input power while delivering three-phase output to the motor. These drives internally rectify the single-phase AC to DC, then invert it back to three-phase AC of variable frequency and voltage .

flowchart TD
    subgraph SystemA["APPROACH A: SINGLE-PHASE INPUT VFD"]
        A["Single-Phase Power In<br>230V/480V 50/60Hz"] --> B["Rectifier<br>Converts AC to DC"]
        B --> C["DC Bus Capacitors<br>Energy Storage / Filtering"]
        C --> D["Inverter<br>IGBTs Create 3-Phase AC"]
        D --> E["Three-Phase Motor<br>Standard Induction"]
        
        F["Control Logic<br>Microprocessor"] --> D
        G["User Interface<br>Speed Control"] --> F
    end
    
    subgraph ProsCons["ADVANTAGES & LIMITATIONS"]
        PA["✓ No External Converter Needed"]
        PB["✓ Variable Speed Control"]
        PC["✗ Requires Derating<br>10HP VFD → 5-7.5HP Output"]
        PD["✗ Harmonic Generation<br>Needs Filters"]
    end
    
    SystemA --> ProsCons
    
    style SystemA fill:#e1f5fe,stroke:#01579b
    style ProsCons fill:#fff9c4,stroke:#f57f17

Key advantage: No external phase converter needed—the VFD does both jobs .

Limitation: Single-phase input VFDs typically require derating. A VFD rated for 10 HP with three-phase input might only handle 5-7.5 HP with single-phase input due to higher ripple current on the DC bus .

Approach B: Phase Converter + Standard VFD + Three-Phase Motor

This approach uses a dedicated phase converter to create balanced three-phase power from a single-phase source, which then feeds a standard three-phase VFD and motor .

flowchart TD
    subgraph SystemB["APPROACH B: PHASE CONVERTER + STANDARD VFD"]
        A["Single-Phase Power In"] --> B["Phase Converter<br>Rotary or Static"]
        
        subgraph Rotary["ROTARY CONVERTER DETAIL"]
            R1["Idler Motor<br>3-Phase Motor Runs as Generator"]
            R2["Capacitor Bank<br>For Voltage Balancing"]
            R1 <--> R2
        end
        
        B --> C["Generated Three-Phase Power<br>May Have Imperfect Balance"]
        C --> D["Standard Three-Phase VFD<br>Input: 3-Phase, Output: Variable"]
        D --> E["Three-Phase Motor"]
        
        B -.- Rotary
        
        F["Optional: Multiple Motors<br>Can Run Directly from Converter"]
        C --> F
    end
    
    subgraph ProsCons["ADVANTAGES & LIMITATIONS"]
        PA["✓ Can Use Standard VFDs"]
        PB["✓ Scalable to Multiple Motors"]
        PC["✗ More Complex Installation"]
        PD["✗ Lower Efficiency than Approach A"]
    end
    
    SystemB --> ProsCons
    
    style SystemB fill:#f3e5f5,stroke:#4a148c
    style Rotary fill:#fff3e0,stroke:#e65100
    style ProsCons fill:#fff9c4,stroke:#f57f17

Rotary phase converters use a motor-generator set to create the third phase and are available in sizes up to 40 HP and beyond . They are rugged, reliable, and can power multiple motors.

3.3 Applications in Rural & Agricultural Settings

3.4 Advantages of VFD + Phase Converter Systems

3.5 The Critical Challenge: Harmonic Distortion

While VFD + phase converter systems offer many benefits, they introduce a significant power quality challenge: harmonic distortion.

What Causes Harmonics?

Single-phase VFDs use a diode bridge rectifier to convert AC to DC. This rectifier draws current only at the peaks of the voltage waveform, creating a non-sinusoidal current rich in harmonics—particularly the 3rd, 5th, and 7th orders .

Typical Harmonic Levels (Without Mitigation)

These levels far exceed allowable limits for grid connection in most jurisdictions .

Effects of Harmonic Distortion

• Transformer overheating (eddy current losses)
• Neutral conductor overloading (triplen harmonics add in neutral)
• Capacitor bank failure (resonance with supply inductance)
• Metering errors (some revenue meters inaccurately measure distorted waveforms)
• Interference with communications and sensitive electronics
• Utility penalties or refusal to connect

3.6 Mitigation Strategies for Harmonics

flowchart TD
    subgraph Mitigation["HARMONIC MITIGATION OPTIONS"]
        direction TB
        
        M1["LINE REACTORS<br>3-5% Impedance"] --> E1["Effect: 25-50% Reduction on 5th/7th<br>Minimal Effect on 3rd Harmonic"]
        
        M2["PASSIVE FILTERS<br>Tuned to Specific Harmonics"] --> E2["Effect: 80-90% Reduction All Orders<br>Fixed Tuning, May Resonate"]
        
        M3["ACTIVE FILTERS<br>Dynamic Cancellation"] --> E3["Effect: 90-95%+ Adaptive<br>Expensive, Adjustable"]
        
        M4["MULTI-PULSE DRIVES<br>12 or 18 Pulse"] --> E4["Effect: Eliminates 5th/7th<br>Requires Transformer, Bulky"]
        
        M5["ACTIVE FRONT END<br>IGBT Rectifiers"] --> E5["Effect: <5% THD, Unity PF<br>Highest Cost, Regenerative"]
    end
    
    subgraph Recommendation["RECOMMENDATION BY APPLICATION"]
        R1["Small Systems: Line Reactors + Passive Filter"]
        R2["Pumps/Fans: Passive Filter"]
        R3["Multiple Drives: Active Filter"]
        R4["Critical Power: Active Front End"]
    end
    
    Mitigation --> Recommendation
    
    style Mitigation fill:#e1f5fe,stroke:#01579b
    style Recommendation fill:#e8f5e8,stroke:#1b5e20

A. Line Reactors and DC Link Chokes

The simplest and most cost-effective mitigation is adding line reactors (on the input) and/or DC link chokes (internal to the VFD). These inductors smooth current flow and reduce higher-order harmonics.

Limitation: Reactors alone cannot adequately suppress the 3rd harmonic in single-phase systems .

B. Passive Harmonic Filters

Passive filters use inductors and capacitors tuned to specific frequencies to trap harmonics.

• Tuned filters for 3rd, 5th, 7th can be very effective
• Broadband filters (like the Mirus Lineator 1Q3) reduce THD by up to 10x
• Simple, reliable, no power required
• Fixed tuning—may not adapt to changing loads
• Can cause resonance with system impedance

C. Active Harmonic Filters

Active filters use power electronics to inject cancelling currents in real time, dynamically neutralizing harmonics.

• Excellent performance across all harmonics, including 3rd
• Adapts to varying load conditions
• More expensive and complex
• Requires power and maintenance
• Often used for larger installations or where multiple VFDs share a bus

D. 12-Pulse or 18-Pulse Drives

For larger installations, multi-pulse rectifier configurations cancel lower-order harmonics through phase shifting.

• 12-pulse effectively eliminates 5th and 7th
• 18-pulse also attenuates 11th and 13th
• Requires phase-shifting transformer—bulky and expensive
• Used primarily in large industrial applications

E. Active Front End (AFE) Drives

AFE drives use IGBT-based rectifiers instead of diode bridges, enabling:

• Near-sinusoidal input current (<5% THD)
• Regenerative capability (power back to grid)
• Unity power factor
• Highest cost—justified for large systems or where power quality is critical

3.7 Comparison of Mitigation Options

3.8 Utility Perspective & Compliance

Rural electric cooperatives and utilities are increasingly concerned about harmonic distortion from VFDs and phase converters. Some key considerations:

Utility Requirements (Typical)

• THID < 12% at point of common coupling (often requires filters)
• Individual harmonic limits per IEEE 519 or IEC 61000-3-12
• Pre-installation studies for motors >50 HP
• Some co-ops prohibit phase converters without harmonic filters

3.9 Selection Guide: VFD + Phase Converter vs. Dedicated Single-Phase Motors

3.10 Best Practices for VFD + Phase Converter Installations

1. Assess your load – Is variable speed needed? If yes, VFD approach is best.
2. Check utility requirements – Some co-ops have harmonic limits; discuss before investing.
3. Size appropriately – Single-phase input VFDs require derating; consult manufacturer.
4. Plan for harmonics – Budget for line reactors (minimum) or harmonic filters (preferred).
5. Consider solar integration – Modern solar VFDs can reduce operating costs to near-zero .
6. Think long-term – Three-phase motors are standard; VFDs can be reused if three-phase becomes available.
7. Document compliance – Keep records of harmonic measurements for utility or regulatory purposes.

Part 4: Comparison & Selection Guide

4.1 Technology Comparison Matrix

4.2 Application-Specific Recommendations

For Irrigation Pumps

• Best: VFD + Phase Converter (variable flow saves water/energy)
• Good: Written-Pole (if constant flow acceptable)
• Avoid: Rosenberg (obsolete, parts unavailable)

For Grain Handling (Conveyors, Elevators)

• Best: VFD + Phase Converter (speed matching between equipment)
• Good: Written-Pole (if single speed adequate)
• Avoid: Rosenberg (maintenance intensive)

For Remote/Off-Grid Sites

• Best: Written-Pole (lowest starting current, minimal grid impact)
• Good: VFD + Solar (if renewable energy available)
• Avoid: Rosenberg (requires maintenance access)

For Critical Processes (Water Treatment, Lift Stations)

• Best: Written-Pole (ride-through capability)
• Good: VFD with ride-through configured
• Avoid: Rosenberg (unreliable for critical duty)

4.3 Decision Flowchart

flowchart TD
    Start(["START: Need High Power from Single-Phase?"]) --> Q1{"New Installation or Existing?"}
    
    Q1 -->|New Installation| Q2{"Variable Speed Required?"}
    Q1 -->|Existing Rosenberg Motor| Legacy["Evaluate for Replacement"]
    
    Legacy --> L1["Can you maintain brushes?"]
    L1 -->|Yes - Temporary| Temp["Continue with Maintenance Plan"]
    L1 -->|No - Replace| Q2
    
    Q2 -->|Yes| VFD["VFD + Phase Converter System"]
    Q2 -->|No| Q3{"Weak Grid?<br>Voltage Dip Concerns?"}
    
    Q3 -->|Yes| WP["Written-Pole Motor"]
    Q3 -->|No| Q4{"Budget Available?"}
    
    Q4 -->|Premium| WP2["Written-Pole Motor<br>Best Grid Compatibility"]
    Q4 -->|Standard| VFD2["VFD + Converter with Line Reactors"]
    Q4 -->|Limited| Retro["Consider Used Equipment?<br>⚠️ Not Recommended"]
    
    VFD --> H1["Add Harmonic Filters<br>Check Utility Requirements"]
    VFD2 --> H1
    WP --> H2["Verify 50 HP Limit<br>Order Lead Time 6-12 Weeks"]
    WP2 --> H2
    Retro --> H3["Inspect Thoroughly<br>Plan Future Replacement"]
    
    H1 --> Final(["Implementation"])
    H2 --> Final
    H3 --> Final
    Temp --> Final
    
    style Start fill:#e1f5fe,stroke:#01579b,stroke-width:3px
    style Q1 fill:#fff3e0,stroke:#e65100
    style Q2 fill:#fff3e0,stroke:#e65100
    style Q3 fill:#fff3e0,stroke:#e65100
    style Q4 fill:#fff3e0,stroke:#e65100
    style VFD fill:#f3e5f5,stroke:#4a148c
    style VFD2 fill:#f3e5f5,stroke:#4a148c
    style WP fill:#e8f5e8,stroke:#1b5e20
    style WP2 fill:#e8f5e8,stroke:#1b5e20
    style Legacy fill:#ffebee,stroke:#b71c1c
    style Retro fill:#ffebee,stroke:#b71c1c
    style Temp fill:#fff9c4,stroke:#f57f17
    style Final fill:#fff9c4,stroke:#f57f17,stroke-width:2px

Part 5: References & Further Reading

Standards

Manufacturer Resources

• Precise Power Corporation – Written-Pole Motor documentation
• Mitsubishi Electric – Single-phase input VFD application guides
• Mirus International – Harmonic filter design for single-phase systems
• Phase Converter manufacturers – Rotary and static converter sizing

Part 6: Mobile-Friendly Summary Cards

Mobile Card 1: Rosenberg Motor (Quick Facts)

graph TD
    subgraph Mobile1["📱 ROSENBERG MOTOR - QUICK FACTS"]
        direction TB
        R1["📅 Era: 1910s-1950s"]
        R2["⚡ Power: 5-100 HP"]
        R3["🔧 Type: Repulsion-Start Induction-Run"]
        R4["📈 Start Current: 3-5x FLC"]
        R5["⚠️ Status: OBSOLETE"]
        R6["✅ Pros: High Power, High Torque"]
        R7["❌ Cons: Brushes, Low Efficiency"]
        R8["🎯 Best For: Legacy Equipment Only"]
    end
    
    style Mobile1 fill:#ffebee,stroke:#b71c1c,stroke-width:3px

Mobile Card 2: Written-Pole Motor (Quick Facts)

graph TD
    subgraph Mobile2["📱 WRITTEN-POLE MOTOR - QUICK FACTS"]
        direction TB
        W1["📅 Era: 1990s-Present"]
        W2["⚡ Power: 1-50 HP"]
        W3["🔧 Type: Synchronous with Written Poles"]
        W4["📈 Start Current: 2-3x FLC"]
        W5["✅ Pros: Grid-Friendly, Low Maintenance"]
        W6["❌ Cons: Higher Cost, Fixed Speed"]
        W7["🎯 Best For: Weak Grids, Critical Loads"]
    end
    
    style Mobile2 fill:#e8f5e8,stroke:#1b5e20,stroke-width:3px

Mobile Card 3: VFD + Phase Converter (Quick Facts)

graph TD
    subgraph Mobile3["📱 VFD + PHASE CONVERTER - QUICK FACTS"]
        direction TB
        V1["📅 Era: 1980s-Present"]
        V2["⚡ Power: 1-500+ HP"]
        V3["🔧 Type: Electronic Conversion"]
        V4["📈 Start Current: 1.5-2x FLC"]
        V5["✅ Pros: Variable Speed, Standard Motors"]
        V6["❌ Cons: Harmonics, Needs Filters"]
        V7["🎯 Best For: Pumps, Fans, Variable Loads"]
    end
    
    style Mobile3 fill:#f3e5f5,stroke:#4a148c,stroke-width:3px

📚 References & Further Reading

Standards Organizations

Technical Papers & Articles

Historical References

• General Electric. (1910s-1950s). Induction-Repulsion Motor Technical Bulletins. GE Publication Archives.
• Steinmetz, C.P. (1915). Theory and Calculation of Alternating Current Phenomena. McGraw-Hill.
• Behrend, B.A. (1921). The Induction Motor. McGraw-Hill.

Download complete references document here.

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